Recent advances in the cell engineering have enabled culturing of various animal cells, including human cells. Research on the reconstruction of human tissues or organs using such cells, that is, what is called regenerative medicine, is progressing rapidly.
For example, the most frequently used artificial blood vessels in clinic are those containing non-absorbable polymers, such as GORE-TEX. Unfortunately, with the artificial blood vessels containing non-absorbable polymers, anti-coagulants and the like have to be continuously administered because the artificial blood vessel remain as a foreign body in the body for a long time after grafting. In addition, when such artificial blood vessels are used in children, repeat surgery is disadvantageously required as they grow older. To overcome the situation, regeneration of blood vessel tissue by regenerative medicine has been attempted.
The point of regenerative medicine is whether cells can grow and differentiate into a three-dimensional, living tissue-like structure. In an exemplary method, a substrate is implanted into the patient's body so that cells from the surrounding tissue or organ can enter the substrate and grow and differentiate to regenerate tissue or an organ.
Porous substrates containing bioabsorbable polymers have been proposed as the substrates for regenerative medicine. With the use of such a porous substrate containing a bioabsorbable polymer as the substrate for regenerative medicine, cells enter the voids in the substrate and grow, leading to rapid regeneration of tissue. In addition, such substrates do not need to be removed by repeat surgery as they are decomposed and absorbed in the living body after certain periods of time.
With regard to methods for producing the porous substrates containing bioabsorbable polymers, Patent Literature 1, for example, discloses a method for producing a porous substrate by adding water-soluble particles such as sodium chloride or saccharide particles to a bioabsorbable polymer solution, freeze-drying the resulting solution, and then leaching out and removing the particles by washing with water.
Unfortunately, in the method disclosed in Patent Literature 1, dispersing the particles in the bioabsorbable polymer solution is difficult, so that precipitation of the particles causes the resulting porous substrate to have a non-uniform pore size distribution. In addition, complete removal of the particles requires a complicated process. Furthermore, disadvantageously, it is substantially impossible to produce a porous substrate when the bioabsorbable polymer solution has high viscosity.
To overcome the situation, methods for producing a porous substrate by a phase separation process have been proposed. These methods include mixing good and poor solvents for a bioabsorbable polymer so as to form a uniform phase, followed by cooling to give a porous body. For example, Patent Literature 2 discloses a method for producing a porous substrate including dissolving a polymer containing a lactide-caprolactone copolymer into a mixed solution of good and poor solvents for the polymer, followed by cooling. Patent Literature 3 discloses a method for producing a porous substrate including adding polylactic acid to a mixed solution containing an organic solvent capable of dissolving the polylactic acid, an organic solvent not dissolving the polylactic acid, and water, followed by heating at 40° C. to 100° C. to dissolve the polylactic acid, and further followed by cooling.
In the porous substrates containing bioabsorbable polymers, control of properties such as pore size and bulk density is extremely important from the standpoint of mechanical strength to serve as a tissue regeneration scaffold, bioabsorption behavior, permeability to cells, supply of nutrition to cells entering the substrate, and the like. In the phase separation process, the pore size of the resulting porous substrate can be adjusted by adjustment of the mixing ratio between the good solvent and the poor solvent. However, the adjustment of the pore size of the porous substrate in this manner greatly varies the bulk density of the resulting porous substrate. Specifically, for a porous substrate with a large pore size, the ratio of the poor solvent has to be high. This makes the ratio of the good solvent relatively low, so that the resulting porous substrate has a small bulk density. Conversely, for a porous substrate with a small pore size, the ratio of the good solvent is set high and that of the poor solvent set low, so that the resulting porous substrate has a large bulk density. Therefore, unfortunately, a porous substrate having a different pore size but the same bulk density is very difficult to produce by the phase separation process. Furthermore, the phase separation process requires that the good solvent and the poor solvent are compatible with each other. When water, which is easy to handle, is selected as the poor solvent, there are only limited choices of good solvents such as 1,4-dioxane, N-methylpyrrolidone, and dimethyl sulfoxide. These solvents are highly toxic to the living body, and thus steps for completely removing the solvents from the porous substrates are required for clinical application. This disadvantageously makes the production process complicated.